As linux-lts moved to the 5.10 version, all official kernels of Arch Linux now support zstd compressed initramfs images, so mkinitcpio is switching to zstd compressed images by default with version 30, which is currently on [testing].
If, for any reason, you are using a kernel version prior to 5.9, make sure to change mkinitcpio.conf COMPRESSION to use one of the compressors supported, like gzip, otherwise you will not be able to boot images generated by mkinitcpio.